Color television circuits are known which (1) derive a third color difference signal, such as "G-Y" from first and second color difference signals, such as "R-Y" and "B-Y", which (2) provide a relatively low output impedance (i.e., buffered outputs) for all three color difference signals and which (3) feature minimal differential delay among the output signals.
An example of a conventional color difference signal matrix and buffer circuit which provides each and every one of the three (3) foregoing features is described by S. A. Roth in U.S. Pat. No. 3,719,772 entitled COLOR TELEVISION MATRIXING CIRCUIT which issued Mar. 6, 1973.
The Roth circuit includes a source of R-Y and B-Y color difference signals to be matrixed. The R-Y signal is applied to an operational amplifier connected in a non-inverting configuration thereby providing a "buffered" (i.e., low impedance output) for the R-Y signal with a gain determined by a resistive feedback network for the operational amplifier. The B-Y signal is similarly applied to an operational amplifier connected in a non-inverting configuration with a feedback network for determining the B-Y gain and providing a low output impedance.
The G--Y signal is produced by applying the R-Y and B-Y input signals via respective input resistors to the summing (i.e., inverting) input of a third operational amplifier connected in an inverting configuration. The third operational amplifier provides a weighted and inverted sum of the R-Y and B-Y input signals determined by the two input resistors and the feedback resistor thereby generating (matrixing) a G-Y output signal which is buffered (i.e., provided at a low impedance) by the third operational amplifier.
In the Roth circuit, differential delay errors are minimized because the delay of the inverting differential amplifier, which provides matrixing of the G-Y signal, is offset or compensated for by the similar delays of the two non-inverting differential amplifiers which buffer the R-Y and B-Y signals.